package com.hs.util;

/**
 * twitter的snowflake算法 雪花算法，用于生成long型ID ID为64位Long。 从高位算起，第一部分为 毫米时间戳， 占 64 - 22
 * = 42 位。其次数据中心5位，机器标识5位，最后同一毫秒内的序列号12位。
 */
public class SnowFlake {

    /**
     * 起始的时间戳 2018-08-20
     */
    private static final long START_TIMESTAMP = 1534694400000L;

    /**
     * 每一部分占用的位数
     */
    private static final long SEQUENCE_BIT = 12; // 序列号占用的位数
    private static final long MACHINE_BIT = 5; // 机器标识占用的位数
    private static final long DATA_CENTER_BIT = 5;// 数据中心占用的位数

    /**
     * 每一部分的最大值
     */
    private static final long MAX_DATA_CENTER_NUM = (1L << DATA_CENTER_BIT) - 1;
    private static final long MAX_MACHINE_NUM = (1L << MACHINE_BIT) - 1;
    private static final long MAX_SEQUENCE = (1L << SEQUENCE_BIT) - 1;

    /**
     * 每一部分向左的位移
     */
    private static final long MACHINE_LEFT = SEQUENCE_BIT;
    private static final long DATA_CENTER_LEFT = SEQUENCE_BIT + MACHINE_BIT;
    private static final long TIMESTAMP_LEFT = DATA_CENTER_LEFT + DATA_CENTER_BIT;

    private final long dataCenterId; // 数据中心
    private final long machineId; // 机器标识
    private long sequence = 0l; // 序列号
    private long lastTimestamp = -1L;// 上一次时间戳

    /**
     * constructor
     *
     * @param dataCenterId
     * @param machineId
     */
    public SnowFlake(long dataCenterId, long machineId) {
        if (dataCenterId > MAX_DATA_CENTER_NUM || dataCenterId < 0) {
            throw new IllegalArgumentException("dataCenterId can't be greater than MAX_DATA_CENTER_NUM or less than 0");
        }
        if (machineId > MAX_MACHINE_NUM || machineId < 0) {
            throw new IllegalArgumentException("machineId can't be greater than MAX_MACHINE_NUM or less than 0");
        }
        this.dataCenterId = dataCenterId;
        this.machineId = machineId;
    }

    /**
     * 产生下一个ID
     *
     * @return
     */
    public synchronized long nextId() {
        long curStamp = getNewTimestamp();
        if (curStamp < lastTimestamp) {
            throw new RuntimeException("Clock moved backwards.  Refusing to generate id");
        }

        if (curStamp == lastTimestamp) {
            // 相同毫秒内，序列号自增
            sequence++;
            // 同一毫秒的序列数已经达到最大，则等待下一毫秒，且重制sequence
            if (sequence > MAX_SEQUENCE) {
                curStamp = getNextMill();
                sequence = 0L;
            }
        } else {
            // 不同毫秒内，序列号置为0
            sequence = 0L;
        }

        lastTimestamp = curStamp;
        return (curStamp - START_TIMESTAMP) << TIMESTAMP_LEFT // 时间戳部分
                | dataCenterId << DATA_CENTER_LEFT // 数据中心部分
                | machineId << MACHINE_LEFT // 机器标识部分
                | sequence; // 序列号部分
    }

    private long getNextMill() {
        long mill = getNewTimestamp();
        while (mill <= lastTimestamp) {
            mill = getNewTimestamp();
        }
        return mill;
    }

    private long getNewTimestamp() {
        return System.currentTimeMillis();
    }

    public static void main(String[] args) {
        SnowFlake snowFlake = new SnowFlake(1, 1);
        String l = String.valueOf(snowFlake.nextId());
        System.out.println(l);
    }

}